Connections between cells of the developing squid as revealed by electrophysiological methods.

cell impaled during electrical measurements.'6 The dye, Niagara Sky Blue 6B, was used in a 3.5-4% aqueous solution. Impalement of an embryonic cell was accomplished by bringing the microelectrode into contact with it and then passing a pulse of inward current (about 10-8 A; 1 sec duration). Entry was signaled by appearance of the resting potential, and with dye-filled electrodes the same current pulse usually ejected enough dye to stain the impaled cell. If the pulse failed to cause entry, the dye diffused away in extracellular spaces leaving no mark. To verify the identification of the marked cells, experimental embryos were fixed overnight at 00C in a fixativel6 acidified to retain the dye marks: glutaraldehyde (25% solution), 2.5 ml; acrolein, 0.2 ml; 0.1 M acetate buffer, pH 4.0, 5 ml; 2 X artificial sulfate-free sea water, 2.5 ml. After dehydration in ethanol they were cleared in propylene oxide and embedded in epon. The hardness of the epon was adjusted so that serial sections could be cut at 5 A with a steel knife." Adenovirus infection of monkey kidney cells in tissue culture is abortive; the early T antigens are formed efficiently and typical adenovirus cytopathic changes occur, but only limited amounts of viral structural antigens, morphologic particles, and infectious virus are produced.1-' In cells infected with SV40 virus, this block is eliminated; the viral antigens and infectivity are formed with efficiency com-parable to that in human cells.'-3 for of describes the application of the nucleic acid homology technique to determine whether SV40 enhancement acted at a step before or after Ad. 7 DNA synthesis, and whether SV40 DNA could.be demonstrated in E46+.